The desire to maintain a youthful appearance by reducing wrinkles in the skin is an important issue in human society. Many techniques have been designed to achieve this goal. One of the techniques is, for example, skin rejuvenation, particularly methods that involve denaturation of collagen, such as thermal denaturation of collagen. Although some therapeutic applications are possible, the main area of interest is in the cosmetic, or non-therapeutic, field. The efficacy of such methods in the skin depends on several factors, such as thermal and mechanical load on the tissue, age of the person undergoing the treatment, anatomical distribution of collagen inside the skin, diseases of the skin, environmental exposure, skin type, etc. At present the parameters of the denaturation-based skin rejuvenation treatments are often based on trial and error.
An increasing number of these skin treatments and skin treatment systems are intended for use by consumers rather than medical professionals. These treatment systems are non-invasive—they create an effect beneath the surface of the skin without having to physically penetrate the epidermis. However, such home-use systems raise new concerns, such as concerns relating to safety and treatment efficacy. This is particularly important when the light source for performing the treatment is a laser, and incorrect operation of such a laser can result in scarring or burning of the skin at locations where the laser light passes through the skin layers.
Damage to the epidermis, for example, is highly undesirable because this may lead to complications and health risks to the person being treated, as well as social downtime. If superficial lesions are created above the dermis, petechiae (micro-bleeding) may occur due to micro-rupturing of capillaries, resulting in reduced efficacy and an increase in side effects. The formation of new collagen for the purpose of skin rejuvenation will occur if the collagen is denaturized. The efficacy of a thermal treatment for collagen denaturation, and the subsequent collagen remodeling, are high only if the temperature inside the dermis exceeds a critical temperature of 65 degrees C. Treatments are therefore provided which direct considerable amounts of energy to raise the temperature of the skin above the threshold required for denaturation.
Treatment of skin regions where no collagen is present may result in over-treatment. Assuming collagen is present, changes in the length of the collagen fibers continue during denaturation until the fibers have shrunk to a minimum length—thermal treatment beyond this phase is over-treatment because the collagen structure is completely disrupted. Over-treatment may result in unnecessary damage to surrounding tissues and other side effects without promoting any rejuvenation and tightening effect.
Because the effectiveness and safety of denaturation of collagen and the resulting skin rejuvenation depend on several factors (as indicated hereinbefore), there is a need to measure the presence of collagen, and to measure the denaturation of collagen inside the skin during the course of treatment.
It is known to use the birefringence of collagen to determine the presence of collagen by tracking the changes seen in the reflected light when the skin is subjected to polarized light, and to repeat such measurements to monitor the progression of the denaturation. This is one of the measurement systems described in WO 2011/112248. Such monitoring systems may be used in combination with many different treatment methods, such as R.F. (radio frequency)-based, U.S. (ultra-sound)-based or laser-based treatment methods. Drawbacks of such known measurement methods reside in that they are relatively complex and time consuming. For example, optical measurement is described in detail in “Quantitative measurements of linear birefringence during heating of native collagen”, Lasers in Surgery and Medicine, Vol. 20, Issue 3, 1997, Pages: 310-318, Duncan J. Maitland and Joseph T. Walsh Jr. The birefringence is measured at a number of points in time by deriving a mean and standard deviation from 60 measurements, using a compensation crystal and a polarizing transmission microscope.
WO 1995/28135 describes a further technique in which the gradual change in intensity and the color change observed when using a polarizing transmission microscope are dealt with. However, the perception of color change and intensity change may differ between different observers, which makes the values subjective. The measurement is not an accurate and quantitative measure of the progression of the treatment, since color change and diminution due to the collagen depend on many factors, including age, skin-optical and thermal properties etc.